This invention relates to electrical noise detection circuits and, more particularly, to circuits responsive to an incoming alternating electrical signal permeated by noise for producing an outgoing electrical signal in which the noise is suppressed. Specifically, the invention is directed to a method and apparatus for blanking noise present in an incoming alternating electrical control signal for producing an outgoing electrical control signal in which no evidence of the noise appears, especially in inherently noisy environments, such as encountered in the use of servomotors in conjunction with the operation of printers, disc drives, and other equipment used in information processing.
In order to facilitate an understanding of the invention, the invention will be described by way of example in connection with printers, namely, daisy wheel printers. The exemplary use of the invention in connection with a daisy wheel printer, however, is by way of illustration only and is not to be interpreted as a limitation of the principles of the invention to daisy wheel printers or to printers generally. As will become clear, the principles which underlie the invention apply generally to control circuits responsive to an incoming alternating electrical control signal in which noise is present for recognizing noise and producing an outgoing electrical control signal which is free from noise.
In a daisy wheel printer, a petal shaped print wheel is rotatably mounted to a carriage. The carriage is reciprocally mounted with respect to a paper feeder so that the reciprocal movement of the carriage is orthogonal with respect to the direction of paper feed, for example, the paper is fed vertically and the carriage is moved horizontally. Rotation of the print wheel, reciprocation of the carriage, and movement of the paper by means of the paper feeder, preconditions the daisy wheel printer for enabling a character to be printed at a preselected print position. Printing requires that the paper be indexed to a line position, the carriage be moved to a character position, and the print wheel be rotated to a selected character, whereupon a hammer means is energized for striking the wheel causing the wheel to be impacted on a ribbon interposed between the wheel and the paper, thereby imprinting a character on the paper.
The print wheel and the carriage are moved to the appropriate positions by respective servomechanisms. In the case of the print wheel, the servomechanism is under the control of a host which selects the characters to be printed. The order in which the characters are printed is also determined by the host which selects the character positions.
It is desirable that the carriage is moved to the proper character position and the print wheel is rotated to the proper character before the hammer means is energized for printing the character on the paper. A long extant problem to which the present invention is addressed is that if the print wheel and/or carriage are not properly positioned by the time that the hammer means is energized, the quality of the printing suffers. That is, if there is a disparity between the position of the print wheel and/or carriage specified by the host and the actual position of the print wheel and/or carriage, imprecise positioning results which causes poor registration of the character with respect to a line on the paper, only a portion of the character being printed, and other unacceptable deviant print characteristics. Furthermore, and perhaps an even more onerous problem is that imprecise positioning of the print wheel and/or carriage can result in damage to or breakage of the print wheel. If the print wheel is rotated to an imprecise position evidenced by the petal shaped cantilevered character imprinting element being offset, that is, slightly out of registration with, the hammer means, impact of the hammer means against the character imprinting element can damage or break the element, thereby necessitating replacement of the print wheel.
In view of the criticality which attaches to properly positioning the print wheel and carriage in order to assure print quality and avoid printer downtime, the respective servomechanisms which position the print wheel and carriage include respective servomotors and closed loop servo control circuits responsive to a command signal from the host for controllably moving the print wheel and carriage to one of various positions with negative feedback for maintaining the position until another command signal is provided. Position feedback is generally provided by respective shaft encoder signals or can be derived from respective tachometer signals, and under optimum conditions the shaft encoders or tachometers must produce signals having sufficient signal-to-noise so that proper positioning can be achieved. However, spurious noise generated by noise sources, such as alternating current power lines, fluorescent light fixtures, other information processing equipment, etc., in the vicinity of the daisy wheel printer, decreases the signal-to-noise ratio of the shaft encoder or tachometer signals. The noise problem is exacerbated by the fact that the brushes of the servomotors themselves generate electrical noise and the daisy wheel printer has other noise sources, such as static electrical charges from the paper feeder. Consequently, noise is present which can adversely affect the signal-to-noise ratio of the signals produced by the shaft encoders or tachometers. As a result, the precision with which the print wheel and carriage are positioned can be substantially impaired.
In the past, the incoming alternating electrical control signals from the shaft encoders or tachometers have been filtered by analog noise filters in the form of resistor-capacitor filters. These resistor-capacitor filters, however, have various disadvantages. The time delays associated with resistor-capacitor filters adversely affect the response of the servo control circuits. Furthermore, the bandwidth of such resistor-capacitor filters is limited. Typically, only very high frequency noise is filtered. If high-frequency noise is filtered, however, the high-frequency content of the signals produced by the shaft encoders or tachometers is also eliminated. This results in rounding the leading and trailing edges in the case where the signals from the shaft encoders or tachometers are rectangular pulses. Unfortunately, sharp leading and trailing edges are needed in order to assure proper operation of digital circuitry. Furthermore, adjustability of the resistor-capacitor filters requires potentiometers or variable capacitors which are expensive.
A more advanced noise recognition and suppression technique is disclosed in a copending patent application of Tri S. Van, U.S. Ser. No. 536,916 filed on Sept. 27, 1983, entitled METHOD AND APPARATUS FOR MASKING NOISE PRESENT IN AN ALTERNATING ELECTRICAL SIGNAL assigned to the same assignee as this application. The embodiments disclosed in the referenced copending application include a hard-wired digital circuit, as well as a microprocessor circuit implementation. Although these embodiments produce acceptable results, the hard-wired digital circuit includes several one shots, which can be unstable, and a number of resistor-capacitor delay circuits. Hence, the hard-wired digital circuit is complex and can be difficult to implement. Furthermore, the microprocessor circuit implementation must handle a significant burden, such that the capability of a single microprocessor can be insufficient, and, therefore, multiple microprocessors can be needed in order to execute all functions required of the control circuit, which adds to the complexity and increases the expense of the microprocessor circuit implementation.
The invention provides cancellation of the effects of noise present in an incoming alternating electrical control signal without the limitations of a resistor-capacitor filter. Furthermore, the invention provides a noise blanking circuit which avoids the use of unstable one shots, as well as resistor-capacitor delay circuits, present in the hard-wired digital circuit disclosed in the referenced copending application and includes fewer circuit elements so as to result in a less complex circuit which can be easily implemented. Like the hard-wired digital circuit disclosed in the referenced copending application, however, the invention provides a circuit which does not place a significant demand on a microprocessor circuit and, therefore, avoids the complexity and expense of a multiple microprocessor implementation. The method and apparatus in accordance with the invention can effectively reduce the impact of noise present in the incoming alternating electrical control signals from the shaft encoders or tachometers included in the servo control circuits for the print wheel and carriage of a daisy wheel printer in order to increase the precision with which the print wheel and carriage are positioned, thereby assuring print quality and avoiding damage to or breakage of the wheel.